Promoting the adhesion of a surface of a titanium material

ABSTRACT

The invention relates to a method for promoting the adhesion of a surface of a titanium material ( 5 ). In order to enable improved, in particular environmentally friendly, adhesion promotion of the surface, an adhesion promoting layer is applied, which comprises nanotubes ( 13 ) that include titanium dioxide (TiO 2 ) and have diameters of 10 to 300 nm. The method also comprises applying an organic material to the adhesion promoting layer ( 11 ) with good adhesion.

The invention relates to a method for promoting the adhesion of a surface of a titanium material and to a vehicle, in particular an aircraft, comprising a titanium material having a surface and an organic material that is associated with the surface with good adhesion.

Promoting the adhesion of a surface of a titanium material is known. The adhesion of organic materials such as adhesive, paint, sealant and/or the like to titanium materials is determined by the condition of the surface of the titanium material. In the case of coatings and/or bonds, the adhesion of the surface of the titanium material can be promoted beforehand, which in the simplest case consists of a cleaning step. It is also known, optionally after the cleaning step, to carry out a physical and/or chemical treatment of the surface of the titanium material, wherein wettability, chemical compatibility and/or mechanical anchoring of the organic material can be influenced. Known processes include mechanical roughening, for example by way of radiation, chemical etching and a formation of adhesive layers by way of chemical and/or electrochemical conversion, for example phosphating and/or anodizing and/or applying coatings. U.S. Pat. No. 4,473,446 discloses a method for treating the surfaces of titanium parts prior to bonding by way of anodizing in a chromic-hydrofluoric acid bath at an anodizing voltage between one volt and 5 volts. U.S. Pat. No. 4,394,224 discloses a method for treating titanium parts or titanium alloy parts so as to create an adhesion-promoting oxide layer. The document includes the steps of applying to and treating the surface with a mixture of aqueous solutions made of sodium hydroxide and hydrogen peroxide, maintaining the applied mixture within a temperature range in which the hydrogen peroxide is relatively stable, and causing an increased rate of oxidation at the surface region. DE 34 27 543 A1 relates to an alkaline bath for treating titanium. The bath is made of an alkali hydroxide, a titanium complexing agent, and a foreign ion complexing agent. U.S. Pat. No. 3,907,609 discloses a chemical conversion process and a composition for producing an adherent conversion coating on titanium and titanium alloys. U.S. Pat. No. 5,814,137 and U.S. Pat. No. 6,037,060 relate to a surface treatment, preferably for titanium and aluminum alloys, for forming a sol gel film covalently bonded on a metal surface to produce strong and durable adhesive bonds between the metal and an organic adhesive without using toxic chemicals and while significantly reducing and/or eliminating rinse water requirements of conventional anodizing and/or etching processes. DE 38 02 043 C1 relates to a method for preparing a metal surface. To this end, so as to prepare for the bonding to plastic, a layer is applied to a metal surface by way of sand blasting using a substance that is composed of 0.1 to 30% by weight optionally silanized, amorphous silicon-containing material having a grain size of less than 1 μm, the remainder being a sand blasting substance having a mean grain size of greater than 1 μm, and this layer subsequently being optionally silanized. DE 10 2006 045 951 A1 relates to a method for chemically modifying and/or activating solid surfaces. Using at least one carrier medium, which is used to supply the surface with energy and with one or more halogen-containing compounds, in the method the halogen-containing compounds are supplied by simultaneously adding organosilicon compounds or silanes or organometallic compounds or silicon hydrides or metal hydrides to the carrier material. WO 2009/015329 A2 relates to a method for forming a perpendicularly oriented titanium nanotube field using electrochemical oxidation. WO 2006/104644 A2 relates to a surface-modified implant, comprising at least one metal-containing surface, comprising a plurality of nanotubes on the surface, wherein the nanotubes have an oxide of the metal-containing surface. U.S. Pat. No. 7,695,767 B2 relates to a method for providing a superhydrophobic surface on a structure, for example airfoils, propellers and/or rotors. The method includes applying a hydrofluoric acid-containing medium to a titanium substrate. US 2010 0028387 A1 relates to a titanium or titanium alloy substrate, coated with a molecular plasma made of deposited polypeptides, wherein the substrate has a nanotubular-structured surface. WO 2009/017945 A2 relates to nanotubularly structured titanium substrates coated with nano-particulate hydroxylapatite (nano-HA).

It is the object of the invention to provide a method, in particular an environmentally friendly method, for pretreating titanium materials, which allows the long-term stable, high-strength joining of organic materials to the titanium materials.

The object is achieved by a method for promoting the adhesion of a surface of a titanium material. The method includes creating an adhesion promoting layer that is fixed to the surface of the titanium material and comprises nanotubes, which include titanium dioxide and have diameters of 10 to 300 nm, in particular 20 to 220 nm, in particular 30 to 180 nm, in particular 40 to 140 nm, in particular 50 to 100 nm, on the surface, and applying an organic material to the adhesion promoting layer comprising the nanotubes with good adhesion. It was found that the combination of the nanotubes having the indicated dimensioning with the organic material allows for particularly long-term stable and high-strength joining of the organic material to the titanium material. The surface can be understood to mean a surface of the titanium material, in particular before applying the adhesion promoting layer.

According to one embodiment of the method, the surface of the titanium material is coated with the organic material. Advantageously, delamination of the organic material from the titanium material can be reliably prevented. The coating is achieved by way of the adhesion promoting layer, which for this purpose is located between and/or in a boundary region between the surface and the organic material.

In a further embodiment of the method, a further material is bonded to the surface comprising the nanotubes by way of an adhesive layer of the organic material. Advantageously, a long-term stable and high-strength bond is obtained between the titanium material and the further material.

In a further embodiment of the method, structural bonding of a component comprising the titanium material to a further component by way of a bond comprising the adhesive layer of the organic material is provided for. Advantageously, structural bonds, in particular for creating a supporting structure, between the titanium material, or the component made of the titanium material, and the further component can be implemented. A bond can be understood to mean in particular an adhesive surface, an adhesive point and/or a plurality of adhesive points.

In a further embodiment of the method, anodic oxidation of the surface of the titanium material so as to generate the nanotubes of the adhesion promoting layer is provided for. The adhesion promoting layer can advantageously be produced in an environmentally friendly manner by way of anodic oxidation.

In a further embodiment of the method, the titanium material comprises an alloy Ti6Al4V, wherein anodic oxidation of the surface takes place in an electrolyte having a composition of 50 to 250 g/l, in particular 120 to 140 g/l, preferably 130 g/l ammonium sulfate, and 0.5 to 10 g/l, in particular 4 to 6 g/l, preferably 5 g/l ammonium fluoride, at a temperature of 10 to 60° C., in particular 20 to 30° C., preferably 25° C., and a voltage of preferably 2 to 50 volts, in particular 10 to 20 volts for 5 to 480 minutes, in particular 20 to 40 minutes, preferably 30 minutes. It was found that the indicated electrolytes advantageously have re-dissolution properties, wherein the nanotubes can advantageously be produced in the desired dimension using the indicated parameters. It is possible to create a layer thickness of 100 to 750 nm and a pore diameter of approximately 15 to 80 nm.

The object is also achieved by a vehicle, in particular an aircraft, comprising a titanium material having a surface and an organic material that is associated with the surface with good adhesion. According to the invention, an adhesion promoting layer is disposed between the surface and the organic material, the layer being fixed to the surface of the titanium material, associated with the organic material with good adhesion and comprises nanotubes include contain titanium dioxide and have diameters of 10 to 300 nm, in particular 20 to 220 nm, in particular 30 to 180 nm, in particular 40 to 140 nm, preferably 50 to 100 nm. Advantageously long-term stable and high-strength joining of the organic material to the titanium material by way of the adhesion promoting layer is achieved.

In one exemplary embodiment of the vehicle, the vehicle comprises a component comprising the titanium material and the adhesion promoting layer, the component being structurally bonded to a further component of the vehicle by way of an adhesive layer of the organic material. Advantageously a supporting structure of the vehicle can be created using the structural bond, the structure satisfying the requirements in terms of corrosion resistance and/or stability.

According to a further exemplary embodiment of the vehicle, the adhesion promoting layer has a thickness of 100 nm to 10 μm, in particular 200 nm to 1 μm, in particular 250 to 800 nm, in particular 280 to 600 nm, in particular 300 to 500 nm. Advantageously, a long-term stable adhesion promoting layer can be implemented in the indicated thicknesses.

Additional advantages, characteristics and details will be apparent from the description, which describes in detail at least one exemplary embodiment—optionally with reference to the drawing. The described and/or depicted features form the subject matter of the invention, either alone or in any arbitrary useful combination, optionally also regardless of the claims, and can in particular also be the subject matter of one or more separate applications. Identical parts, similar parts and/or parts with equivalent functions have been denoted by the same reference numerals.

In the drawings:

FIG. 1 is a partially illustrated aircraft comprising a structural bond of a component with a titanium material and a further component;

FIG. 2 is a side view of an electron microscopy image of an anodization layer;

FIG. 3 is a top view onto the anodization layer shown in FIG. 2;

FIG. 4 is a top view onto a joining point between a component comprising a titanium material and a further component after a butt-joint test; and

FIG. 5 is a comparison chart of an aging test by way of a wedge test using three different adhesion promoting methods.

FIG. 1 is a partial view of an aircraft 1 comprising a component 3, which comprises and/or consists of a titanium material 5. A titanium material can be understood to mean titanium and/or a titanium alloy. The component 3 is structurally bonded to a further component 7 by way of an adhesive layer 9, in particular a glued layer of an adhesive. The further component 7 can comprise any arbitrary material, in particular an aluminum material, a fiber-reinforced plastic material or likewise a titanium material. The adhesive layer 9 is fixed to the component 3 by way of an adhesion promoting layer 11. The adhesion promoting layer 11 comprises nanotubes 13, which comprise titanium dioxide.

The creation of the nanotubes 13 of the adhesion promoting layer 11 on the titanium material 5 will be described in greater detail hereafter based on a specific example.

The component 3 can be a metal sheet made of the alloy Ti6Al4V, for example. This is immersed in an electrolyte composed of 130 g/l ammonium sulfate and 5 g/l ammonium fluoride. The electrolyte is advantageously free of hydrofluoric acid. Using Ti6Al4V as the counter electrode, anodization is carried out at a temperature of 25° C. using a voltage of 10 to 20 volts for 30 minutes. Advantageously, a regularly structured, porous oxide layer having a thickness of approximately 400 to 500 nm is created. The upper open pores, which advantageously form the nanotubes 13, have pore diameters of approximately 40 to 80 nm. FIG. 2 shows a corresponding result.

FIG. 2 shows an electron microscopy image of the adhesion promoting layer 11 in a so-called cryo fracture, wherein the adhesion promoting layer 11 comprises and/or consists of the nanotubes 13, so-called TiO₂ nanotubes, which is to say the anodization layer.

FIG. 3 shows a top view onto the adhesion promoting layer 11, which is shown in FIG. 2, of the anodization layer on the titanium material 5 Ti6Al4V.

Two adhesion tests were carried out so as to check the bonding strength of the adhesive layer 9 and evaluate the described method.

Adhesion was tested by way of a butt-joint test according to ISO 4624. For this purpose, a Ti6Al4V metal sheet, which is to say the component 3, for example, was pretreated by way of the previously described method and bonded to a chromic acid-anodized aluminum plunger, for example the further component 7. An epoxy adhesive, a two-component construction adhesive, known by the brand name Scotch-Weld® DP 490, available from 3M, was used as the glue for the adhesive layer 9. This adhesive has a tensile shear resistance of 26 MPa (at 23° C. and with pickled aluminum). Advantageously, purely cohesive failure in the adhesive, which is to say in the adhesive layer 9, was found in the tested samples, which is shown in greater detail in FIG. 4.

So as to evaluate aging resistance of the pre-treatment conducted by way of the previously described method, a wedge test according to DIN 65448 was carried out. At a climate of 50° C. and 95% relative humidity, crack propagation comparable to the so-called NaTESi method was achieved using the previously described anodization method. The NaTESi method is disclosed in DE 34 27 543 A1, for example. The NaTESi method was developed for high-strength structural bonds and can be used as a reference method with respect to strength.

Compared to a commercial titanium pretreatment, for example in the form of alkaline pickling, especially for bonds, the previously described method results in significantly less crack propagation, which is apparent from FIG. 5.

FIG. 5 shows a graph 15, wherein a time between 0 and 1000 h is plotted on an x-axis 17 and a crack length between 20 mm and 100 mm is plotted on a y-axis 19. In chart 15, a total of three crack curves, which is to say a first crack curve 21 for alkaline pickling, a second crack curve 23 for the previously described method according to the invention, and a third crack curve 25 for the NaTESi method are plotted. It is apparent that the previously described method according to the invention achieves the values of the NaTESi method and is consequently equivalent to the same.

Long-term stable nanostructured titanium surfaces can advantageously be created by way of the previously described method in a particularly environmentally friendly manner, which provide a suitable substrate for long-term stable and/or high-strength organic coatings. It is conceivable to apply only a coating, instead of the adhesive layer 9, to the adhesion promoting layer 11.

The method according to the invention advantageously results in long-term stable nanostructured titanium surfaces. This structuring, which is to say the plurality of nanotubes 13, makes structural bonds exhibiting long-term stability possible, in particular in aircraft construction, which advantageously makes new design concepts possible (see FIG. 1). Likewise, delamination of paints, in particular on titanium rivets, can advantageously be reliably prevented by way of the pre-treatment, wherein advantageously the costs for re-painting can be avoided and optionally maintenance intervals can be saved.

By way of the method according to the invention, advantageously the adhesion promoting layer 11 can be created on the component 3 comprising the titanium material 5, wherein this layer can advantageously be provided with coatings that have good adhesion and are made of organic materials such as adhesive, paint, sealant and the like, wherein high adhesion and long-term stability of the corresponding coatings and/or bonds are achieved.

The basis of the method is the creation of a stable oxide layer on the titanium surface of the titanium material 5 by way of anodic oxidation in an electrolyte, wherein this electrolyte advantageously comprises constituents that are not harmful to the environment.

According to the invention, electrolytes are used for the titanium pre-treatment, by way of which porous surface morphologies can be produced on titanium in a targeted manner. All electrolytes that exhibit re-dissolution properties and are therefore suitable for creating pore structures are suited, wherein the diameters thereof range between 10 and 300 nm, 20 and 220 nm, in particular 30 and 180 nm, in particular 40 to 140 nm, preferably 50 to 100 nm.

Suitable thicknesses of the oxide layers, which is to say of the adhesion promoting layer 11, range between 100 nm and 10 μm, in particular 200 nm and 1 μm, in particular 250 and 800 nm, in particular 280 and 600 nm, preferably 300 to 500 nm.

The method according to the invention advantageously does not comprise or use any hydrofluoric acid, which is to say is free of hydrofluoric acid, wherein advantageously a lower potential for risk exists.

LIST OF REFERENCE NUMERALS

-   1 aircraft -   3 component -   5 titanium material -   7 component -   9 adhesive layer -   11 adhesion promoting layer -   13 nanotubes -   15 graph -   17 x-axis -   19 y-axis -   21 first crack curve -   23 second crack curve -   25 third crack curve 

1. A method of promoting the adhesion of a surface of a titanium material, the method comprising: creating an adhesion promoting layer that is fixed to the surface of the titanium material and comprises nanotubes, wherein the nanotubes include titanium dioxide (TiO₂) and have diameters in a range of 10 nm to 300 nm; and applying an organic material to the adhesion promoting layer with good adhesion.
 2. The method according to claim 1, further comprising coating the surface of the titanium material with the organic material.
 3. The method according to claim 1, further comprising bonding a further material to the surface comprising the nanotubes by way of an adhesive layer of the organic material.
 4. The method according to claim 3, further comprising structurally bonding a component comprising the titanium material to a further component by way of a bond that comprises the adhesive layer of the organic material.
 5. The method according to claim 1, further comprising anodically oxidizing the surface of the titanium material so as to create the nanotubes of the adhesion promoting layer.
 6. The method according to claim 1, further comprising anodically oxidizing, without using hydrofluoric acid, the surface of the titanium material so as to create the nanotubes of the adhesion promoting layer.
 7. The method according to claim 1, further comprising anodically oxidizing the surface in an electrolyte that comprises 50 g/l to 250 g/l ammonium sulfate and 0.5 to 10 g/l ammonium fluoride, at a temperature in a range of 10° C. to 60° C., at a voltage in a range of 2 volts to 50 volts, for a time in a range of 5 minutes to 480 minutes.
 8. The method according to claim 7, wherein the titanium material is made of Ti6Al4V.
 9. The method of claim 7, wherein the electrolyte comprises 120 g/l to 140 g/l ammonium sulfate and 4 g/l to 6 g/l ammonium fluoride.
 10. A vehicle comprising: a titanium material having a surface and an organic material that is associated with the surface with good adhesion; and an adhesion promoting layer disposed between the surface and the organic material, the adhesion promoting layer being fixed to the surface of the titanium material and associated with the organic material with good adhesion, the adhesion promoting layer comprising nanotubes that include titanium dioxide (TiO₂) and have diameters in a range of 10 nm to 300 nm.
 11. The vehicle according to claim 10, wherein the vehicle comprises a component comprising the titanium material and the adhesion promoting layer, the component being structurally bonded to a further component of the vehicle by way of an adhesive layer of the organic material.
 12. The vehicle according to claim 10, wherein the adhesion promoting layer has a thickness in a range selected from a group of ranges comprising: 100 nm to 10 μm, 200 nm to 1 μm; 250 nm to 800 nm; 280 nm to 600 nm; and 300 nm to 500 nm.
 13. The vehicle according to claim 10, wherein the titanium material is made of Ti6Al4V.
 14. The vehicle according to claim 10, wherein the vehicle is an aircraft.
 15. The vehicle according to claim 10, wherein the diameters are in a range selected from a group of ranges comprising: 20 nm to 220 nm; 30 nm to 180 nm; 40 to 140 nm; and 50 nm to 100 nm.
 16. The method according to claim 7, wherein the temperature is in a range 20° C. to 30° C., the voltage is in a range of 10 volts to 20 volts, and the time is in a range of 20 minutes to 40 minutes.
 17. The method according to claim 1, further comprising anodically oxidizing the surface in an electrolyte that comprises ammonium sulfate and ammonium fluoride.
 18. The method according to claim 17, wherein the electrolyte comprises 50 g/l to 250 g/l ammonium sulfate and 0.5 g/l to 10 g/l ammonium fluoride.
 19. The method according to claim 17, wherein the surface is anodically oxidized at a temperature in a range of 10° C. to 60° C., at a voltage in a range of 2 volts to 50 volts, for a time in a range of 5 minutes to 480 minutes.
 20. An electrolyte to produce a porous surface of a titanium material, the electrolyte comprising: an ammonium sulfate in a range of 50 g/l to 250 g/l; and an ammonium fluoride of 0.5 to 10 g/l.
 21. The electrolyte according to claim 20, wherein the ammonium sulfate is in a range of 120 g/l to 140 g/l and the ammonium fluoride is in a range of 4 g/l to 6 g/l. 